A wing tip device is attached to the outboard end of a wing to reduce induced drag on the wing. In the case of e.g. an aircraft wing this can lead to improved fuel efficiency and reduced carbon emissions. Wing tip devices can take a variety of forms.
A winglet is a wing-like element that extends from the wing tip. A winglet may extend upwardly or downwardly from the wing tip. NASA TN D-8260 entitled “A Design Approach and Selected Wind-Tunnel Results at High Subsonic Speeds for Wing-Tip Mounted Winglets”; Whitcomb, R. T.; 1976 describes a wing tip device having a lower winglet (extending downwardly from the wing tip) forward of an upper winglet (extending upwardly from the wing tip). The sizing of these tip devices are recommended in NASA T M 81230 entitled “Effect of Winglets on the Induced Drag of Ideal Wing Shapes”; R T Jones and T A Lasinski 1980.
A wing tip fence is a special form of wing tip device that extends vertically both above and below the wing tip. U.S. Pat. No. 4,714,215 describes a wing tip fence.
Another example of a wing tip device is a non-planar wing tip extension, i.e. it extends out of the plane of the wing to which it is attached. A winglet may be considered to be a particular example of a non-planar wing tip extension. US 2002/0162917 describes a non-planar wing tip extension having continuously increasing curvature of local dihedral, continuously increasing sweepback (at both leading and tailing edges), and continuously decreasing chord in the outboard direction.
A winglet may include a substantially planar portion joined to the wing tip by a curved transition portion to form a blended winglet, such as described in U.S. Pat. No. 5,348,253. The transition portion has a constant radius of curvature. The specified blend is said to reduce interference drag effects at the wing tip.
Alternatively, a winglet may include a substantially planar portion joined to the wing tip by a non-planar wing tip extension portion, such as described in WO 2008/061739. The non-planar wing tip extension portion has increasing curvature of local dihedral in the outboard direction. The wing tip extension portion is said to further reduce interference drag effects compared to a blended winglet with a constant radius transition.
Another example of a wing tip device is a substantially planar wing tip extension, such as the raked wing tip described in U.S. Pat. No. 6,089,502, which does not extend substantially out of the plane of the wing. Raked wing tips can achieve similar drag reduction performance to winglets.
Span constraints on aircraft, due to e.g. airport compatibility gate limits or aircraft category flying constraints, mean that winglets or non-planar wing tip extensions, rather than raked wing tips, may need to be adopted in order to reduce induced drag on the wing. Since winglets (and non-planar wing tip extensions more generally) extend out of the plane of the wing to which they are attached, an effective increase in the wing aspect ratio can be achieved (which reduces the vortex-induced drag on the wing) without significantly increasing wing span.
The problem of span constraints is traditionally solved by optimising the wing span of the aircraft in the (full fuel load) ground shape where the span constraints are applicable. However, due to the bend induced on the wing shape from aeroelastic effects during flight, the wing span of the resulting flight shape is usually reduced and is therefore no longer optimal. This traditional approach therefore carries a performance shortfall. This problem becomes even more appreciable with greater use of relatively flexible wings to reduce structural weight, which tend to result in increased wing bending under aerodynamic load when compared to more rigid designs.